1. Field of the Invention
The invention relates to water processing, and, in particular, it is concerned with a method for softening water by way of an electrochemical treatment, and an electrolyzer for carrying out the method.
2. Description of the Related Art
Known in the art is a method for softening water by adding reagents, e.g. lime in the form of lime milk to water being softened. Hardness salts turn to insoluble form and can be separated from water being softened in the form of a precipitate by setting or filtering (cf. Construction Norms and Rules. Water Supply. Outside Networks and Installation (in Russian). SNiP 2.04.84. 1985, Moscow, pp. 108-109). The known method does not allow a desired degree of softening of water to be achieved. The residual hardness can only be lowered to 0.5-1 mg-eq./l using auxiliary methods such as adding soda and heating water to 35.degree.-40.degree. C. In addition, the known method involves the use of reagent adding equipment, mixers, suspended bed clarifiers and apparatuses for stabilizing treatment of water and coagulants. In other words, the known method for softening water using chemicals calls for sophisticated equipment including a large number of individual components.
Also known in the art is a method for softening water by way of electrochemical treatment, comprising causing water to pass in the form of two streams through a membrane electrolyzer, one stream being caused to pass through an anode chamber to produce anolyte, the other stream being caused to pass through a cathode chamber to produce catholyte from which precipitating hardness salts are separated, combining catholyte and anolyte together and using the joint flow as the end product (cf. Jr. "Vodosnabzhenie i sanitarnaya tekhnika", 1982, No. 4, pp. 7-8). The prior art method can only lower hardness of water to 0.8-1.2 mg-eq./l, i.e. the method is essentially a method of prepurification of water prior to, e.g. a sodium-cationite method of water softening, and water being softened is supplied, after its treatment in the membrane electrolyzer, to an ion-exchange apparatus in which it is treated using the sodium-cationite method.
This prior art method calls for the use of chemically pure reagent NaCl, HCl so that the method is rather expensive. In addition, a high water consumption for washing is required. This method is generally used for softening water with a low hardness.
Known in the art is an electrolyzer for softening water, comprising a casing divided by a membrane into flow-through chambers: an anode chamber having an anode accommodated therein and a cathode chamber having a cathode accommodated therein, the anode and cathode being in the form of plates, pressed against the membrane on either side, and separated therefrom with respective electrically insulating layers, the anode and cathode and the electrically insulating layers having perforations (SU, A, 882944).
The prior art electrolyzer provides a poor quality of purification in the anode chamber as the anode prevents cations from passing from the anode chamber to the cathode chamber, the positively charged ions being repelled from the anode, i.e. such electrolyzers are substantially pH electrocoagulators. In additon, manufacture and assembly of the electrolyzer call for a high accuracy of coaxial perforation holes since if they are misaligned the holes are partly covered so as to reduce the working surface area of the membrane.
Most intensive electrolysis occurs inside the perforation holes in view of minimum electrical resistance which is especially pronounced with a large thickness of electrodes, e.g. when graphite electrodes are used. However, this results in clogging of the membrane in the zones of the holes with electrolysis products so as to lower reliability of the electrolyzer in operation.